Reactor for the formation of material on a substrate

ABSTRACT

A reactor for the formation of material on a substrate has a reactor tube with a bottom that is heated by a heating element that does not substantially increase the temperature of the remainder of the tube.

United. States Patent Kohler et al.

REACTOR FOR THE FORMATION OF MATERIAL ON A SUBSTRATE Willem A. Kohler,Los Gatos; Joseph A. Flood, Mountain View, both of Calif.

lnventors:

Assignee: Falrchild Camera and Instrument Corporation, Syosset, LongIsland, NY.

Filed: Nov. 5, 1970 App1.No.: 87,315

Related U.S. Application Data Continuation of Ser. No. 757,874, Sept. 6,1968, abandoned.

U.S. Cl. ..118/48, 219/398 Int. Cl. ..C23c 13/08 Field 01 Search ..1l8/48-49.5, 64,

[15] 3,658,032 [451 Apr. 25, 1972 [56] References Cited UNITED STATESPATENTS 2,369,561 2/1945 Grisdale ..l18/48 2,674,809 4/1954 'Meinhoffer....2l9/388 UX 2,828,225 3/ 1958 Goetzel et al. ...1l8/49.1 UX 3,190,2626/1965 Bakish et a1. ..1 18/48 3,343,518 9/1967 Westeren et a1. 118/4953,473,510 10/1969 Sheng et a1. ..1l8/49.5

Primary Examiner-Morris Kapian Attorney-Roger S. Borovoy and Alan H.MacPherson 57] ABSTRACT A reactor for the formation of material on asubstrate has a reactor tube with a bottom that is heated by a heatingelement that does not substantially increase the temperature of theremainder of the tube.

4 Claims, 2 Drawing Flgtl'es PATENTEUAPR 25 [972 3,658 O32 WILLEM A.KOHLER JOSEPH A. FL

M ii

ATTO/PA/Ei BACKGROUND OF THE INVENTION 1. Prior Art In the semiconductorart, growing semiconductor material (e.g., silicon)onto a substrate is awell-known process. This process is most commonly performed in what isreferred to as a radio (or high) frequency (RF )reactor. Such reactorsemploy a non-conductive reactor tube surrounded in part by an RF coilthat is energized by a source, such as an RF generator. Within thereactor tube is a boat of an electrically conductive material forholding the substrate. The RF coil creates an electromagnetic field thatinteracts with the conductive boat to heat both it and the substratesthereon while the walls of the reactor tube remain relatively coolcompared to the boat. The substrate is heated to a temperaturesufficiently high to cause gases passing through the reactor tube in theproximity of the substrateto decompose, or to react with other gaseswithin the tube or both, and form a semiconductor material on thesubstrate. A typical example for epitaxial silicon growth is the overallreaction SiCl,,+2H Si+4HCl. For the deposition of a typical dielectricfilm, such as silicon nitride, a typical reaction is 3SiCl, 4NI'I SiN.,+l2l-lCl. Also very commonly used is silane,SiH instead of silicontetrachloride in the above mentioned reactions.

Because the RF reactor is designed so that the reactor walls surroundingthe boat are of an electrically nonconductive material, and remainrelatively cool while the boat itself is heated, this reactor hascertain advantages for which it has been widely accepted in thesemiconductor industry. However, such a rector also has drawbacks.

First, the RF coil, along with its energy source (such as an RFgenerator) and elaborate apparatus to provide cooling (such as awater-cooled jacket and an extensive system of cooling coils) isexpensive both to install and to operate. For example, the installationcost often exceeds $50,000. Moreover, because of the large power lossassociated with inductive RF-type heating, the reactor is inefficient,requiring approximately 130 to 175 kilowatts of input power to operatethe RF generator, which induces only about 30 to 50 kilowatts of powerin the RF coil.

Second, the field developed by the RF coils is not uniform over thelength and width of the reactor and is particularly nonuniform acrossthe width of the boat holding one or more substrates. Consequently, thesemiconductor substrates are not uniformly heated. This nonuniformheating results in the nonuniform deposition of material upon thesubstrates, which is particularly undesirable in the fabrication ofsolid-state device such as transistors, diodes and integrated circuits.In such devices, small thickness variations of deposited material in themicron or even angstrom range can cause significant variations in deviceoperating characteristics.

In addition, the reproducibility of the results obtained from theRF-type reactor in solid-state device applications has not beensatisfactory. More particularly, it has been observed that theelectrical characteristics of material formed using an RF reactor canvary in accordance with the location of the substrate on the boat. Onesolution to the reproducibility problem has been to limit the useablelength of the reactor tube to specific dimensions. This approach,however, further restricts the potential efficiency of a particularreactor.

Another type of reactor used in the semiconductor industry is thehot-tube furnace. Here, heating elements surround a quartz tube withinwhich is placed a boat for carrying substrates. The tube is heated fromall sides and consequently the boat is similarly heated to a temperaturehigh enough to cause decomposition of gases flowing within the tube.This method eliminates the expensive equipment needed by the RF reactor,and does not have the loss in efficiency associated with inductiveheating. However, it is considered unsatisfactory because the walls ofthe reactor tube are heated above the temperature of decomposition ofgases passing through the tube, so that unwanted deposits form on thetube wall, due to a chemical reaction that occurs when the gases passover the hot walls. Also, unwanted impurities from the heating elementdiffuse through the hot quartz wall and are added to the materialdeposited onto the substrates, thereby detrimentally affecting theoperating quality of subsequent devices comprising the substrates.

SUMMARY OF THE INVENTION This invention overcomes many of the problemsof the prior art reactors. The reactor of this invention produces a moreuniform temperature gradient throughout the boat than the prior art RFreactors and eliminates both undesired deposits along the reactor tubewalls and unwanted impurities in the deposited material, a major problemwith the hot-tube fur- ,nace. In addition, the reactor of this inventionis inexpensive,

between material deposited on substrates on the same boat are reduced.

In accordance with this invention, a reactor tube is provided with botha curved top portion and a flat bottom portion adapted for holding aflat support upon which semiconductor substrates are placed. Inlets forpermitting gases to flow into the reactor tube and an outlet forpermitting the gases to leave the reactor tube are placed on the ends ofthe tube. Beneath the bottom portion of the tube is placed a segmentedheating unit containing a plurality of electrically isolated,individually controlled heating elements. These heating elements rest ona contoured insulating support.

Upon applying power to the heating elements, the flat bottom of the tubebecomes heated and in turn heats the substrates on the boat resu'ng onthis bottom. However, because the tube wall is further away from theheating elements than the bottom portion, and because of the coolingeffect of the flowing gases, the wall remains relatively cool comparedto the bottom portion and the boat resting thereon. Consequently,decomposition of the gases, or a reaction therebetween, or both, takesplaces only in the vicinity of the boat and thus little, if any, depositis formed on the tube wall. Also, the cooler wall prevents impuritiesfrom diffusing therethrough onto the substrates.

Furthermore, the boat fully covers the heated bottom portion, therebypreventing the possibility of any impurities from the heated bottombeing formed onto the substrates carried by the boat. However, few, ifany, impurities ever collect at the bottom portion, because the heatingelements underneath the bottom are imbedded in an insulating material,which reduces the possibility of any impurities moving between theheating elements and the bottom portion, and because the presence of theboat itself overlying the bottom prevents deposition from above onto thebottom portion. Each of the heating elements can be individuallyadjusted so that the temperature gradients appearing along the axis ofthe tube can be precisely controlled to obtain a desired profile.

This invention will be more fully understood in light of the followingdetailed description taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective ofthe reactor; and, FIG. 2 is a sectional view taken along the lines 2-2of FIG. 1 with the components in an assembled relationship.

DETAILED DESCRIPTION OF THE DRAWING The reactor comprises a reactor tube12, a heating unit 40, and housing 60. The reactor tube 12 has a tubularportion 14 with a flat bottom 16 a plurality of inlets 18 and 20 for theentry of gases into the tubular portion 14 and a outlet 22 for theexiting of gases from the tubular portion 14. The tubular portion 14 iscurved in cross section, shown in FIG. 2 as semicircular, with bottom 16bridging the cross section in a substantially diametrical or chordrelationship. However, for some applications, portion 14 may berectangular in shape. Typically, tubular portion 14 has a length ofapproximately 26 inches and an outside diameter of approximately 2%inches.

At one end of tubular portion 14 is a tapered section 24 having an openend 26 adapted to receive end cap 28 having a plurality of ears 30 and32 for receiving a plurality of springs 34 and 36 attached at theirother ends to microswitches 37 and 38, which in turn control safetyrelays on a control panel (not shown). Microswitches 37 and 38 areoperated when the pressure conditions in the reactor tube are too highor end cap 28 is displaced. The switches 37 and 38 will'then causesafety circuits to be energized to prevent the flowing of all gasesexcept an inert gas, such as N,, through the inlets. Switches 37 and 38also set off ,an alarm unless the end cap is properly seated to seal thereactor tube during the operating condition. The opening 26 is necessaryto permit the insertion of a member 52 (commonly referred to as aboat).that is adapted to receive substrates 54 spaced along the entirelength of boat 52. The width of substrate 54 is only limited by thewidth of the boat 52. In a typical case of an approximately 2 /2 inchwide boat, the substrate may be up to approximately 2% inches wide. Theboat 52 comprises a material that is heat conductive, has a low vaporpressure at high temperature, is chemically inert, and is nondegassing.For example, a suitable boat material comprises graphite coated withsilicon or silicon carbide. The boat 52 has a width dimension that isonly slightly less than that of flat portion 16 so that flat portion 16is covered and little, if any, growth or deposit occurs along itslength. The boat 52 may experience some growth or deposit thereon butsuch growth is removable.

Heating unit 40 is located below flat portion 16 and is formed by aplurality of resistance heating elements, such as elements 42, 44 and46. Each heating element is connected to a separate and independentlycontrolled power source, for example, power sources 48, 50 and 51, whichare of a type well known in the art and can be thermo-couple controlledwith manually adjustable settings. This feature enables each of theheating elements 42,44 and 46 to be separately controlled at differenttemperatures, and permits one selectively to vary the temperatureprofile along the length of the boat. Also it is within the scope of theinvention to change the number of individually controlled heatingelements, thereby providing great flexibility in selecting the desiredtemperature profile. The heating elements 42, 44 and 46 together have anoverall length that is substantially identical in length with that ofboat 52. Preferably the heating elements 42, 44 and 46 are resistanceheating elements (such as Kanthal) embedded in an insulating and holdingmaterial 40, such as asbestos cement or equivalent material.

The heating elements 42, 44, and 46 having a curved cross section, shownin FIG. 2 as semicircular. Also, the flat bottom portion 16 of tube 12is in diametrical or chord-like relationship to the curved cross sectionof the heating elements 42, 44, and 46. This relationship enables flatportion 16 to be heated at a substantially uniform temperature while thetubular portion 14 is maintained at a substantially lower temperature.It

should be noted that the relative arrangement of the curved tubularportion 14, the flat bottom 16 and the curved heating unit 40, providesa very effective thermo dynamic balance whereby the flat portion 16 maybe maintained at a precisely controlled temperature profile, uniformacross its width.

More specifically, heat losses such as losses due to radiation orconvection or both, are more likely to occur along the edges than in thecenter of the boat 52. However, because the edges of boat 52 are closerto the curved heating unit 40 compared to the center of boat 52, moreintense heat is applied at the edges than at the center, therebycompensating for any heat losses along the edges. Also, because thecurved portion 14 of tube 12 is farther from the heating unit 40 thanthe flat bottom 16 and because of the cooling effect of gases flowingthrough the tube 12, the curved portion 14 is kept at a substantiallylower temperature than that of the flat bottom 16. Also, the heatingelements 42 and 46 may be individually adjusted to vary the heat appliedto the ends of the boat 54 compared to the heat applied to the middle ofthe boat by heating element 44. The curved cross section of tubularportion 14 and heating unit 40 is shown as semicircular, because this isa particularly convenient shape for ease of manufacturing, and alsobecause with respect to the heating unit 40, this shape provides aparticularly uniform temperature distribution. However, other curvedshapes may be used, depending upon a particular application. Further,the selected shape of curved tubular portion 14 may be different fromthat of heating unit 40, without departing from the scope of theinvention.

The heating unit 40 along with reactor tube 12 are positioned in housing60 by an insulator lining 62 having a curved cross section in conformitywith heating unit 40. A pair of spacers 64 and 66 position the reactortube 12 with respect to the lining 62. A cover 68, only part of which isshown, encloses the unit.

The ends 70 and 71 of the reactor consist of transite plates 74 and 75approximately A inch thick, and thick fire bricks 76 and 77approximately 1 inch thick. Although shown as separate pieces in theexploded drawing of FIG. 1, the ends 70 and 71 are in fact an integratedpart of the reactor wherein the latter has a flat surface at both ends.

In operation, taking a typical case for the deposition of siliconnitride (Si -,N,), gases such as silane (Sil-L) or silicon tetrachloride(SiCl are supplied to inlet 18 along with a carrier gas such as hydrogen(H or nitrogen (N,). When employing silane, a typical flow rate is 24cubic centimeters per minute of silane with hydrogen (H as the carriergas being delivered at the rate of 10 liters per minute. The other inlet20 is also supplied with one of the reaction gases such an ammonia (N11which when employing silane and hydrogen at the rates indicated, istypically supplied at a rate of 24 cubic centimeters per minute.

Prior to the entry of the reacting gasses through inlets l8 and 20, theresistance heating elements 42, 44 and 46 are energized by power sources48, 50 and 51 which have been set to maintain the desired temperature.In the case of silicon nitride deposition with silane, this temperaturewould be approximately 7l0 C. With respect to silicon tetrachloride, ata temperature of approximately 850 C. is necessary. With the heatingelements energized, the boat 52 and substrates 54 thereon are heated toa temperature sufficient to cause the gases flowing thereover todecompose, interact, and form a deposit on the substrate. For example,silane can react with ammonia to form a deposit of silicon nitride.Because the tubular portion 14 is farther away from the heating unit 40than flat bottom 16 and because of the cooling efiect of the flowinggases, the tubular portion 14 remains at a temperature lower than thetemperature of decomposition of the gases, and thus there is little, ifany, deposit on portion 14. The excess gases and reaction products exitvia outlet 22.

In summary, the reactor of this invention eliminates the need for acomplicated RF generator, RF coil, and cooling mechanism. The reactorhas an installation cost that is more than an order of magnitude lessthan that of the presently available RF-type reactors. The main flowrate of the gases employed in this reactor may be one third of thatemployed in an RF reactor of the same capacity. Because of both theunique location of the curved heating elements relative to the flatreactor bottom and the use of a plurality of individually controlledheating elements, the resulting deposition of material on the substratesis of substantially uniform thickness and may be readily reproduced.

The reactor of the invention'has a wide range of applications tosemiconductor materials'and it has been proven particularly effectivewith respect to the deposition of silicon nitride and epitaxial silicon.For example, the material that can be deposited onto substrates withinthe reactor may be of single crystalline, polycrystalline, or amorphousstructure, depending upon such considerations as the gases used, thegrowth parameters such as flow rate and boat temperature, the type ofmaterial deposited, and the surface condition of the substrate itself.

For single crystalline growth, semiconductor material such as silicon,germanium, gallium arsenide, and others can be epitaxially grown onsingle crystalline semiconductor substrates, or, in special cases, onsingle crystalline or amorphous insulating substrates, such as silicondioxide and aluminum oxide with or without a coating (e.g., a 2,000 A.film of tantalum).

For polycrystalline and amorphous growth, on the other hand, the abovementioned semiconductor materials, and insulating materials (such assilicon-nitride, aluminum-oxide, or silicon dioxide) can be deposited ona very wide range of single crystalline, polycrystalline, or amorphoussubstrates of semiconducting, insulating, or metallic material. Examplesof semiconducting substrates comprise silicon, germanium, and galliumarsenide. Examples of insulating substrates comprise silicon dioxide,germanium dioxide, and aluminum oxide. Examples of metallic substratescomprise molybdenum and tantalum. The substrate may also comprise acombination of the above mentioned materials. Also, the substrates maycomprise one or more of the above mentioned materials with a film, ormore than one film, deposited over it.

Further, using organic vapors, insulating films can be deposited bythermal polymerization. Moreover, metals can be deposited ontosubstrates within this reactor by using pyrolytic decomposition of metalorganic vapors or by the reduction of vapors of inorganic compounds.

What is claimed is:

1. A reactor for controlling the location of the decomposition orreaction of gases therein in order to form selected material on thesurface of semiconductor substrates located at the flat bottom portionof the reactor tube but not on the curved top portion without the needof a cooling unit, while providing protection from alkali ioncontamination, the reactor comprising:

a reactor tube of generally semicircular cross section and having acurved top portion and a flat bottom portion;

a member adapted to support semiconductor substrates located upon saidflat bottom portion, the member being almost as wide as the flatportion;

inlet means for enabling gases to flow selectively into said reactortube and outlet means for enabling gases to flow out of said reactortube; and

sealing means for keeping said reactor tube operation, except for gasflowing in said means;

a plurality of individually controlled resistance heating membersadapted to establish a temperature gradient along the longitudinal axisof said reactor and located beneath said bottom portion for selectivelyheating said bottom portion and said member but not said top portion sothat said member is heated to a relatively high temperature, while saidtop portion remains at a relatively low temperature and gases flowingthrough said reactor tube decompose or react or both at said member butnot along said top portion and selected material is formed upon thesurface of the substrates but not upon said top portion; and

each said heating member having an upwardly directed, open ended,semicircular cross section and supporting said flat bottom portion ofthe reactor on said open end.

2. The reactor of claim 1 wherein said heating elements comprisefilaments embedded in insulating material.

3. The reactor of claim 2 wherein saidinsulating material comprisesasbestos.

,4. The reactor of claim 1 wherein graphite.

gas tight during inlet and outlet said member comprises =0 i t t

1. A reactor for controlling the location of the decomposition or reaction of gases therein in order to form selected material on the surface of semiconductor substrates located at the flat bottom portion of the reactor tube but not on the curved top portion without the need of a cooling unit, while providing protection from alkali ion contamination, the reactor comprising: a reactor tube of generally semicircular cross section and having a curved top portion and a flat bottom portion; a member adapted to support semiconductor substrates located upon said flat bottom portion, the member being almost as wide as the flat portion; inlet means for enabling gases to flow selectively into said reactor tube and outlet means for enabling gases to flow out of said reactor tube; and sealing means for keeping said reactor tube gas tight during operation, except for gas flowing in said inlet and outlet means; a plurality of individually controlled resistance heating members adapted to establish a temperature gradient along the longitudinal axis of said reactor and located beneath said bottom portion for selectively heating said bottom portion and said member but not said top portion so that said member is heated to a relatively high temperature, while said top portion remains at a relatively low temperature and gases flowing through said reactor tube decompose or react or both at said member but not along said top portion and selected material is formed upon the surface of the substrates but not upon said top portion; and each said heating member having an upwardly directed, open ended, semicircular cross section and supporting said flat bottom portion of the reactor on said open end.
 2. The reactor of claim 1 wherein said heating elements comprise filaments embedded in insulating material.
 3. The reactor of claim 2 wherein said insulating material comprises asbestos.
 4. The reactor of claim 1 wherein said member comprises graphite. 